Fibrillation refers to a condition in which muscle fibrils enter a state of extremely rapid, small-scale contractions that do not coordinate to affect contraction of the muscle as a whole. When this occurs in the left ventricle, the heart chamber responsible for pumping blood into the arterial vasculature, it is serious and rapidly fatal. When it occurs in the musculature of the atria, it is less immediately serious and not necessarily fatal. It is still important to treat atrial fibrillation, however, for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to stroke or cerebral vascular accidents resulting from emboli forming in the left atrium. Although drug therapy, in-hospital cardioversion, and implantable cardioverter/defibrillators are acceptable treatment modalities for atrial fibrillation, a curative approach such as ablation therapy offers a number of advantages to certain patients, including convenience and greater efficacy.
Electrical ablation therapy treats cardiac arrhythmias by destroying myocardial tissue involved in the initiation or maintenance of the tachyarrhythmia. Ablation is most often accomplished by delivering radiofrequency electrical energy to a catheter electrode that has been placed next to the tissue to be destroyed. One way that the technique has been employed in order to treat atrial fibrillation is to identify ectopic sites or reentrant pathways electrophysiologically by mapping the electrical activation of the atria. Once candidate sites for ablation are identified they are ablated by the application of radiofrequency energy. Recent evidence has shown that a high percentage of paroxysms of atrial fibrillation are initiated by trains of rapid discharges originating from the pulmonary veins of the left atrium. Accordingly, catheter techniques have been developed for ablating these sites with radiofrequency energy applied from within the pulmonary veins, but electrophysiological mapping of such sites is difficult. Alternatively, another ablation technique involves the production of a circumferential ablation lesion within a pulmonary vein in order to block the conduction pathway from the vein to the left atrium. An effective circumferential lesion must be completely circular, however, and this means that the ablation device must be precisely centered within the vein or ostium, which may be difficult to accomplish. Furthermore, a common complication of this procedure is pulmonary venous stenosis resulting from scarring within the pulmonary vein with variable clinical consequences.